1. Field of the Invention
The present invention relates to high density memory devices, and particularly to memory devices in which multiple levels of memory cells are arranged to provide a three-dimensional 3D array.
2. Description of Related Art
As critical dimensions of devices in integrated circuits shrink to the limits of common memory cell technologies, designers have been looking to techniques for stacking multiple planes of memory cells to achieve greater storage capacity, and to achieve lower costs per bit. For example, thin film transistor techniques are applied to charge trapping memory technologies in Lai, et al., “A Multi-Layer Stackable Thin-Film Transistor (TFT) NAND-Type Flash Memory”, IEEE Int'l Electron Devices Meeting, 11-13 Dec. 2006; and in Jung et al., “Three Dimensionally Stacked NAND Flash Memory Technology Using Stacking Single Crystal Si Layers on ILD and TANOS Structure for Beyond 30 nm Node”, IEEE Int'l Electron Devices Meeting, 11-13 Dec. 2006. See also, U.S. Pat. No. 7,473,589, entitled STACKED THIN FILM TRANSISTOR, NON-VOLATILE MEMORY DEVICES AND METHODS FOR FABRTICATING SAME, by Lai et al., issued Jan. 6, 2009.
Also, cross-point array techniques have been applied for anti-fuse memory in Johnson et al., “512-Mb PROM With a Three-Dimensional Array of Diode/Anti-fuse Memory Cells” IEEE J. of Solid-State Circuits, vol. 38, no. 11, Nov. 2003. In the design described in Johnson et al., multiple layers of word lines and bit lines are provided, with memory elements at the cross-points. The memory elements comprise a p+ polysilicon anode connected to a word line, and an n-polysilicon cathode connected to a bit line, with the anode and cathode separated by anti-fuse material.
In the processes described in Lai, et al., Jung, et al. and Johnson et al., there are several critical lithography steps for each memory layer. Thus, the number of critical lithography steps needed to manufacture the device is multiplied by the number of layers that are implemented. Critical lithography steps are expensive to implement and increase the layout area of the devices as area is added to accommodate inherent misalignment among the critical masks.
As the need for higher and higher memory capacity in integrated circuit memory devices continues to increase, it is desirable to provide a structure for three-dimensional integrated circuit memory with a low manufacturing cost, including reliable, very small memory elements that can be erased and programmed.